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 * AGREEMENT. IF YOU ARE NOT WILLING TO BE BOUND BY IT, SELECT THE "DECLINE"
 * BUTTON AT THE BOTTOM OF THIS PAGE. Specification: JSR-354 Money and Currency
 * API ("Specification") Copyright (c) 2012-2014, Credit Suisse All rights
 * reserved.
 */
package javax.money.convert;

import javax.money.CurrencySupplier;
import javax.money.CurrencyUnit;
import javax.money.NumberValue;
import java.io.Serializable;
import java.util.List;

This class models an exchange rate, which defines the factor the numeric value of a base amount in some currency
'A' must be multiplied
to get the corresponding amount in the terminating currency 'B'. Hereby

an exchange rate always models one rate from a base (source) to a term (target) CurrencyUnit.
an exchange rate is always bound to a rate type, which typically matches
the data source of the conversion data, e.g. different credit card providers
may use different rates for the same conversion.
an exchange rate may restrict its validity. In most of the use cases a
rates' validity will be well defined, but it is also possible that the data
provider is not able to support the rate's validity, leaving it undefined-
an exchange rate has a provider, which is responsible for defining the
rate. A provider hereby may be, but must not be the same as the rate's data
source.
an exchange rate can be a direct rate, where its factor is
represented by a single conversion step. Or it can model a derived
rate, where multiple conversion steps are required to define the overall
base/term conversion. In case of derived rates the chained rates define the
overall factor, by multiplying the individual chain rate factors. Of course,
this also requires that each subsequent rate's base currency in the chain
does match the previous term currency (and vice versa):
Whereas the factor should be directly implied by the format rate chain
for derived rates, this is obviously not the case for the validity range,
since rates can have a undefined validity range. Nevertheless in many cases
also the validity range can (but must not) be derived from the rate chain.
Finally a conversion rate is always unidirectional. There might be cases where the reciprocal value of getFactor()} matches the correct reverse rate. But in most use cases the reverse rate either has a different rate (not equal to the reciprocal value), or might not be defined at all. Therefore for reversing a ExchangeRate one must access an ExchangeRateProvider and query for the reverse rate.

 The class also implements Comparable to allow sorting of multiple exchange rates using the following sorting order; 

Exchange rate type
Exchange rate provider
base currency
term currency

 Finally ExchangeRate is modeled as an immutable and thread safe type. Also exchange rates are Serializable, hereby serializing in the following form and order: 

The base CurrencyUnit 
The target CurrencyUnit 
The factor (NumberValue)
The ConversionContext 
The rate chain


Implementation Specification
Implementations of this interface

must be Comparable(with ExchangeRate)
must implement equals/hashCode considering #getBaseCurrency, #getCurrency,
#getFactor and #getContext.
should be thread-safe
should be serializable
should provide a fluent builder API for constructing new rate instances easily.


Author:
Werner Keil, Anatole Tresch
See Also:
Wikipedia:
 * Exchange Rate (Quotations)/**
 * This class models an exchange rate, which defines the factor the numeric value of a base amount in some currency
 * 'A' must be multiplied
 * to get the corresponding amount in the terminating currency 'B'. Hereby
 * <ul>
 * <li>an exchange rate always models one rate from a base (source) to a term
 * (target) {@link CurrencyUnit}.</li>
 * <li>an exchange rate is always bound to a rate type, which typically matches
 * the data source of the conversion data, e.g. different credit card providers
 * may use different rates for the same conversion.</li>
 * <li>an exchange rate may restrict its validity. In most of the use cases a
 * rates' validity will be well defined, but it is also possible that the data
 * provider is not able to support the rate's validity, leaving it undefined-</li>
 * <li>an exchange rate has a provider, which is responsible for defining the
 * rate. A provider hereby may be, but must not be the same as the rate's data
 * source.</li>
 * <li>an exchange rate can be a <i>direct</i> rate, where its factor is
 * represented by a single conversion step. Or it can model a <i>derived</i>
 * rate, where multiple conversion steps are required to define the overall
 * base/term conversion. In case of derived rates the chained rates define the
 * overall factor, by multiplying the individual chain rate factors. Of course,
 * this also requires that each subsequent rate's base currency in the chain
 * does match the previous term currency (and vice versa):</li>
 * <li>Whereas the factor should be directly implied by the format rate chain
 * for derived rates, this is obviously not the case for the validity range,
 * since rates can have a undefined validity range. Nevertheless in many cases
 * also the validity range can (but must not) be derived from the rate chain.</li>
 * <li>Finally a conversion rate is always unidirectional. There might be cases
 * where the reciprocal value of {@link #getFactor()}} matches the correct reverse
 * rate. But in most use cases the reverse rate either has a different rate (not
 * equal to the reciprocal value), or might not be defined at all. Therefore for
 * reversing a ExchangeRate one must access an {@link ExchangeRateProvider} and
 * query for the reverse rate.</li>
 * </ul>
 * <p/>
 * The class also implements {@link Comparable} to allow sorting of multiple
 * exchange rates using the following sorting order;
 * <ul>
 * <li>Exchange rate type</li>
 * <li>Exchange rate provider</li>
 * <li>base currency</li>
 * <li>term currency</li>
 * </ul>
 * <p/>
 * Finally ExchangeRate is modeled as an immutable and thread safe type. Also
 * exchange rates are {@link Serializable}, hereby serializing in the following
 * form and order:
 * <ul>
 * <li>The base {@link CurrencyUnit}
 * <li>The target {@link CurrencyUnit}
 * <li>The factor (NumberValue)
 * <li>The {@link ConversionContext}
 * <li>The rate chain
 * </ul>
 * <p/>
 * <h3>Implementation Specification</h3>
 * <p>Implementations of this interface
 * <ul>
 * <li>must be Comparable(with {@code ExchangeRate})</li>
 * <li>must implement equals/hashCode considering #getBaseCurrency, #getCurrency,
 * #getFactor and #getContext.</li>
 * <li>should be thread-safe</li>
 * <li>should be serializable</li>
 * <li>should provide a fluent builder API for constructing new rate instances easily.</li>
 * </ul>
 * </ul></p>
 *
 * @author Werner Keil
 * @author Anatole Tresch
 * @see <a
 * href="https://en.wikipedia.org/wiki/Exchange_rate#Quotations">Wikipedia:
 * Exchange Rate (Quotations)</a>
 */
public interface ExchangeRate extends CurrencySupplier {

    
Access the ConversionContext of ExchangeRate. 
Returns:
the conversion context, never null./**
     * Access the {@link ConversionContext} of {@link ExchangeRate}.
     *
     * @return the conversion context, never null.
     */
    ConversionContext getContext();

    
Get the base (source) CurrencyUnit. 
Returns:
the base CurrencyUnit./**
     * Get the base (source) {@link CurrencyUnit}.
     *
     * @return the base {@link CurrencyUnit}.
     */
    CurrencyUnit getBaseCurrency();

    
Get the term (target) CurrencyUnit. 
Returns:
the term CurrencyUnit./**
     * Get the term (target) {@link CurrencyUnit}.
     *
     * @return the term {@link CurrencyUnit}.
     */
    @Override
    CurrencyUnit getCurrency();

    
Access the rate's bid factor.
Returns:
the bid factor for this exchange rate, or null./**
     * Access the rate's bid factor.
     *
     * @return the bid factor for this exchange rate, or {@code null}.
     */
    NumberValue getFactor();

    
Access the chain of exchange rates.
Returns:
the chain of rates, in case of a derived rate, this may be
several instances. For a direct exchange rate, this equals to
new ExchangeRate[]{this}./**
     * Access the chain of exchange rates.
     *
     * @return the chain of rates, in case of a derived rate, this may be
     * several instances. For a direct exchange rate, this equals to
     * <code>new ExchangeRate[]{this}</code>.
     */
    List<ExchangeRate> getExchangeRateChain();

    
Allows to evaluate if this exchange rate is a derived exchange rate.
Derived exchange rates are defined by an ordered list of subconversions
with intermediate steps, whereas a direct conversion is possible in one
steps.
 This method always returns true, if the chain contains more than one rate. Direct rates, have also a chain, but with exact one rate. 
Returns:
true, if the exchange rate is derived./**
     * Allows to evaluate if this exchange rate is a derived exchange rate.
     * Derived exchange rates are defined by an ordered list of subconversions
     * with intermediate steps, whereas a direct conversion is possible in one
     * steps.
     * <p>
     * This method always returns {@code true}, if the chain contains more than
     * one rate. Direct rates, have also a chain, but with exact one rate.
     *
     * @return true, if the exchange rate is derived.
     */
    default boolean isDerived() {
        return getExchangeRateChain().size() > 1;
    }

}